Hi Wily;
an interesting question.
It might appear that there are perhaps several theories concerning this......
Historically, around the late 1980's and early 1990's, there began to be a collection of "add-on" techniques that were being taught along with reiki. They felt it was logical to put this together in a class and teach it as a separate level. Most of it is taken from other non-reiki things such as wicca, new agisms, shamanism, magick, neurolinguistic programming/hypnosis, past life therapy work, Barbara Brennan's stuff, etc. It is not reiki but is often taught along with it.
There are some that think that there was also a financial incentive for doing this so that the student has to pay twice for the master level. I cannot comment on another's motivation regarding this. I can say that there are many that have been taught this material as "traditional reiki" or taught this system that do not know that it was added pretty late in the reiki game and is unecessary.
There are some that think that some teachers, not having lived the system didn't have the experience to actually teach level III in a meaningful and instructive way and thus didn't have enough "material". Teaching the add-on stuff was a way to cover that and provide filler for the price.
There are some, more cynically minded, who think that it was a way to differentiate oneself as a teacher by offering "new and improved" (so called "advance reiki" - although it is neither "advanced", nor technically reiki) reiki so that you could differentiate yourself as a teacher and thus generate more students and revenue stream.
What ever the theory, none of the so called "master practitioner", level IIIa stuff is necessary in the least.
Hope that clarifies.
Good luck with the teacher search.
Take gentle care;
Vinny
